An instance of this structure defines the low-level routines
used to allocate and use mutexes.

Usually, the default mutex implementations provided by SQLite are
sufficient, however the application has the option of substituting a custom
implementation for specialized deployments or systems for which SQLite
does not provide a suitable implementation. In this case, the application
creates and populates an instance of this structure to pass
to sqlite3_config() along with the SQLITE_CONFIG_MUTEX option.
Additionally, an instance of this structure can be used as an
output variable when querying the system for the current mutex
implementation, using the SQLITE_CONFIG_GETMUTEX option.

R-49959-19447:[The xMutexInit method defined by this structure is invoked as
part of system initialization by the sqlite3_initialize() function.]R-28676-32830:[The xMutexInit routine is called by SQLite exactly once for each
effective call to sqlite3_initialize().]

R-43713-34954:[The xMutexEnd method defined by this structure is invoked as
part of system shutdown by the sqlite3_shutdown() function.]
The
implementation of this method is expected to release all outstanding
resources obtained by the mutex methods implementation, especially
those obtained by the xMutexInit method. R-06483-27112:[The xMutexEnd()
interface is invoked exactly once for each call to sqlite3_shutdown().]

The only difference is that the public sqlite3_XXX functions enumerated
above silently ignore any invocations that pass a NULL pointer instead
of a valid mutex handle. The implementations of the methods defined
by this structure are not required to handle this case, the results
of passing a NULL pointer instead of a valid mutex handle are undefined
(i.e. it is acceptable to provide an implementation that segfaults if
it is passed a NULL pointer).

The xMutexInit() method must be threadsafe. It must be harmless to
invoke xMutexInit() multiple times within the same process and without
intervening calls to xMutexEnd(). Second and subsequent calls to
xMutexInit() must be no-ops.

xMutexInit() must not use SQLite memory allocation (sqlite3_malloc()
and its associates). Similarly, xMutexAlloc() must not use SQLite memory
allocation for a static mutex. R-40745-32628:[However xMutexAlloc() may use SQLite
memory allocation for a fast or recursive mutex.]

R-53900-18983:[SQLite will invoke the xMutexEnd() method when sqlite3_shutdown() is
called, but only if the prior call to xMutexInit returned SQLITE_OK.]
If xMutexInit fails in any way, it is expected to clean up after itself
prior to returning.